Here the alternatives often involve branches where some or all must be
satisfied before we can progress.

For example if I want to learn to play a Frank Zappa guitar solo I could
(Fig. 2.2.1)

Transcribe it from the CD. OR

Buy the ``Frank Zappa Guitar Book'' AND Read it from there.

Note the use of arcs to indicate
that one or more nodes must all be satisfied before the parent node is
achieved. To find solutions using an And-Or GRAPH the best first algorithm is
used as a basis with a modification to handle the set of nodes linked by the AND
factor.